This invention relates to magnetic resonance (NMR) techniques. More specifically, this invention relates to the automatic adjustment of the RF transmitter and receiver to the optimal Larmor frequency.
The magnetic resonance phenomenon has been utilized in the past in high resolution magnetic resonance spectroscopy to analyze the structure of chemical compositions. More recently, NMR has been developed as a medical diagnostic modality having applications in imaging the anatomy, as well as in performing in vivo, noninvasive spectroscopic analysis. As is now well known, the NMR phenomenon can be excited within a sample object, such as a human patient, positioned in a homogeneous polarizing magnetic field, B.sub.0, by irradiating the object with radio frequency (RF) energy at the Larmor frequency. In medical diagnostic applications, this is typically accomplished by positioning the patient to be examined in the field of an RF coil having a cylindrical geometry, and energizing the RF coil with an RF power amplifier. Upon cessation of the RF excitation, the same or a different RF coil is used to detect the NMR signals, frequently in the form of spin echoes, emanating from the patient lying within the field of the RF coil. In the course of a complete NMR scan, a plurality of NMR signals are typically observed. The NMR signals are used to derive NMR imaging or spectroscopic information about the patient being imaged or studied.
Before the commencement of each NMR scan, it is common practice to adjust the frequency of the RF transmitter and receiver to insure that the excitation field is at the optimal Larmor frequency. This is necessary to produce the desired image contrast effects in certain NMR measurements and to insure the accuracy of slice selection location. In a human subject, for example, the NMR signal is produced primarily by the protons in water and fat molecules. The Larmor frequency of the protons in these two substances is slightly different and the Larmor frequency of both will vary slightly from patient to patient and at different locations within a patient due to inhomogeneities. In prior NMR scanners, it is common practice to perform a calibration sequence in which an NMR sequence is first executed and the NMR signal is processed to produce on a CRT screen a picture of signal amplitude versus RF frequency. The operator then examines this picture and manually adjusts the frequency of the RF receiver to desired value. For example, the displayed NMR signal may show two peaks, one at the Larmor frequency for fat protons and one at the Larmor frequency for water protons. The operator may choose either frequency, or a frequency therebetween, depending on the particular NMR measurement to be conducted.
A method for automatically calibrating the RF frequency of an MRI scanner is described in U.S. Pat. No. 4,806,866. In this method an NMR signal is acquired from the region of interest and transformed to the frequency domain where the two peaks for fat and water are found by filtering the power spectrum signal and identifying peaks which are spaced apart the proper amount. All peaks are identified by taking the first derivative of the filtered power spectrum and searching for points where the derivative is zero. This method has worked well in most applications to automatically set the MRI scanner frequency to the precise Larmor frequency of fat or water, or to a frequency therebetween, such as the midpoint or certroid. However, it has been discovered that in some applications this method routinely finds the wrong peaks in the filtered power spectrum and does not accurately set the MRI scanner to the proper RF frequency. As a result, the images produced by pulse sequences that are particularly sensitive to RF excitation frequency are unsatisfactory, and the contrast characteristics of images produced with other pulse sequences are varied.